What's in an SPM.mat File?

A while back I attempted to write up a document that summarized everything someone would need to know about using SPM. It was called, I believe, the SPM User's Ultimate Guide: For Alpha Males, By Alpha Males. The title was perhaps a little ambitious, since I stopped updating it after only a few dozen pages. In particular I remember a section where I attempted to tease apart everything contained within the SPM.mat files output after first- and second-level analyses. To me this was the most important section, since complex model setups could be executed fairly easily by someone with a good understanding of Matlab code, but I never completed it.

Fortunately, however, there is someone out there who already vivisected the SPM.mat file and publicly displayed its gruesome remains in the online piazza. Researcher Nikki Sullivan has written an excellent short summary of what each field means, broken down into neat, easily digestible categories. You can find it on her website here, and I have also copied and pasted the information below. It makes an excellent shorthand reference, especially if you've forgotten, for example, where contrast weights are stored in the structure, and don't want to go through the tedium of typing SPM, then SPM.xY, then SPM.xY.VY, and so on.

But if you've forgotten how to rock that body? Girl, ain't no remedy for that.


details on experiment:


SPM.xY.RT - TR length (RT ="repeat time")
SPM.xY.P - matrix of file names
SPM.xY.VY - # of runs x 1 struct array of mapped image volumes (.img file info)
SPM.modality - the data you're using (PET, FMRI, EEG)
SPM.stats.[modality].UFp - critical F-threshold for selecting voxels over which the non-sphericity is estimated (if required) [default: 0.001]
SPM. stats.maxres - maximum number of residual images for smoothness estimation
SPM. stats.maxmem - maximum amount of data processed at a time (in bytes)
SPM.SPMid - version of SPM used
SPM.swd - directory for SPM.mat and img files. default is pwd

basis function:


SPM.xBF.name - name of basis function
SPM.xBF.length - length in seconds of basis
SPM.xBF.order - order of basis set
SPM.xBF.T - number of subdivisions of TR
SPM.xBF.T0 - first time bin (see slice timing)
SPM.xBF.UNITS - options: 'scans'|'secs' for onsets
SPM.xBF.Volterra - order of convolution
SPM.xBF.dt - length of time bin in seconds
SPM.xBF.bf - basis set matrix

Session Stucture:


user-specified covariates/regressors (e.g. motion)
SPM.Sess([sesssion]).C.C - [nxc double] regressor (c#covariates,n#sessions)
SPM.Sess([sesssion]).C.name - names of covariates
conditions & modulators specified - i.e. input structure array
SPM.Sess([sesssion]).U(condition).dt: - time bin length {seconds}
SPM.Sess([sesssion]).U(condition).name - names of conditions
SPM.Sess([sesssion]).U(condition).ons - onset for condition's trials
SPM.Sess([sesssion]).U(condition).dur - duration for condition's trials
SPM.Sess([sesssion]).U(condition).u - (t x j) inputs or stimulus function matrix
SPM.Sess([sesssion]).U(condition).pst - (1 x k) peri-stimulus times (seconds)
parameters/modulators specified
SPM.Sess([sesssion]).U(condition).P - parameter structure/matrix
SPM.Sess([sesssion]).U(condition).P.name - names of modulators/parameters
SPM.Sess([sesssion]).U(condition).P.h - polynomial order of modulating parameter (order of polynomial expansion where 0none)
SPM.Sess([sesssion]).U(condition).P.P - vector of modulating values
SPM.Sess([sesssion]).U(condition).P.P.i - sub-indices of U(i).u for plotting
scan indices for sessions
effect indices for sessions
F Contrast information for input-specific effects
SPM.Sess([sesssion]).Fc.i - F Contrast columns for input-specific effects
SPM.Sess([sesssion]).Fc.name - F Contrast names for input-specific effects
SPM.nscan([session]) - number of scans per session (or if e.g. a t-test, total number of con*.img files)

global variate/normalization details


SPM.xGX.iGXcalc - either "none" or "scaling." for fMRI usually is "none" (no global normalization). if global normalization is "Scaling", see spm_fmri_spm_ui for parameters that will then appear under SPM.xGX.

design matrix information:


SPM.xX.X - Design matrix (raw, not temporally smoothed)
SPM.xX.name - cellstr of parameter names corresponding to columns of design matrix
SPM.xX.I - nScan x 4 matrix of factor level indicators. first column is the replication number. other columns are the levels of each experimental factor. SPM.xX.iH - vector of H partition (indicator variables) indices
SPM.xX.iC - vector of C partition (covariates) indices
SPM.xX.iB - vector of B partition (block effects) indices
SPM.xX.iG - vector of G partition (nuisance variables) indices
SPM.xX.K - cell. low frequency confound: high-pass cutoff (secs)
SPM.xX.K.HParam - low frequency cutoff value
SPM.xX.K.X0 - cosines (high-pass filter)
SPM.xX.W - Optional whitening/weighting matrix used to give weighted least squares estimates (WLS).
  • if not specified spm_spm will set this to whiten the data and render the OLS estimates maximum likelihood i.e. W*W' inv(xVi.V).
SPM.xX.xKXs - space structure for K*W*X, the 'filtered and whitened' design matrix
SPM.xX.xKXs.X - Mtx - matrix of trials and betas (columns) in each trial
SPM.xX.xKXs.tol - tolerance
SPM.xX.xKXs.ds - vectors of singular values
SPM.xX.xKXs.u - u as in X u*diag(ds)*v'
SPM.xX.xKXs.v - v as in X u*diag(ds)*v'
SPM.xX.xKXs.rk - rank
SPM.xX.xKXs.oP - orthogonal projector on X
SPM.xX.xKXs.oPp - orthogonal projector on X'
SPM.xX.xKXs.ups - space in which this one is embedded
SPM.xX.xKXs.sus - subspace
SPM.xX.pKX - pseudoinverse of K*W*X, computed by spm_sp
SPM.xX.Bcov - xX.pKX*xX.V*xX.pKX - variance-covariance matrix of parameter estimates (when multiplied by the voxel-specific hyperparameter ResMS of the parameter estimates (ResSS/xX.trRV ResMS) )
SPM.xX.trRV - trace of R*V
SPM.xX.trRVRV - trace of RVRV
SPM.xX.erdf - effective residual degrees of freedom (trRV^2/trRVRV)
SPM.xX.nKX - design matrix (xX.xKXs.X) scaled for display (see spm_DesMtx('sca',... for details) SPM.xX.sF - cellstr of factor names (columns in SPM.xX.I, i think) SPM.xX.D - struct, design definition SPM.xX.xVi - correlation constraints (see non-sphericity below) SPM.xC - struct. array of covariate info

header info


SPM.P - a matrix of filenames
SPM.V - a vector of structures containing image volume information.
SPM.V.fname - the filename of the image.
SPM.V.dim - the x, y and z dimensions of the volume
SPM.V.dt - A 1x2 array. First element is datatype (see spm_type). The second is 1 or 0 depending on the endian-ness.
SPM.V.mat- a 4x4 affine transformation matrix mapping from voxel coordinates to real world coordinates.
SPM.V.pinfo - plane info for each plane of the volume.
SPM.V.pinfo(1,:) - scale for each plane
SPM.V.pinfo(2,:) - offset for each plane The true voxel intensities of the jth image are given by: val*V.pinfo(1,j) + V.pinfo(2,j)
SPM.V.pinfo(3,:) - offset into image (in bytes).If the size of pinfo is 3x1, then the volume is assumed to be contiguous and each plane has the same scalefactor and offset.

structure describing intrinsic temporal non-sphericity


SPM.xVi.I - typically the same as SPM.xX.I SPM.xVi.h - hyperparameters
SPM.xVi.V xVi.h(1)*xVi.Vi{1} + ...
SPM.xVi.Cy - spatially whitened (used by ReML to estimate h)
SPM.xVi.CY - <(Y - )*(Y - )'>(used by spm_spm_Bayes)
SPM.xVi.Vi - array of non-sphericity components

  • defaults to {speye(size(xX.X,1))} - i.ii.d.
  • specifying a cell array of contraints ((Qi)
  • These contraints invoke spm_reml to estimate hyperparameters assuming V is constant over voxels that provide a high precise estimate of xX.V
SPM.xVi.form - form of non-sphericity (either 'none' or 'AR(1)')
SPM.xX.V - Optional non-sphericity matrix. CCov(e)sigma^2*V.
  • If not specified spm_spm will compute this using a 1st pass to identify signifcant voxels over which to estimate V. A 2nd pass is then used to re-estimate the parameters with WLS and save the ML estimates (unless xX.W is already specified)

filtering information


SPM.K - filter matrix or filtered structure:
  • SPM.K(s) - struct array containing partition-specific specifications
  • SPM.K(s).RT - observation interval in seconds
  • SPM.K(s).row - row of Y constituting block/partitions
  • SPM.K(s).HParam - cut-off period in seconds
  • SPM.K(s).X0 - low frequencies to be removed (DCT)
  • SPM.Y - filtered data matrix

masking information


SPM.xM - Structure containing masking information, or a simple column vector of thresholds corresponding to the images in VY.
SPM.xM.T - [n x 1 double] - Masking index
SPM.xM.TH - nVar x nScan matrix of analysis thresholds, one per image
SPM.xM.I - Implicit masking (0 --> none; 1 --> implicit zero/NaN mask)
SPM.xM.VM - struct array of mapped explicit mask image volumes
SPM.xM.xs - [1x1 struct] cellstr description

design information (self-explanatory names, for once) 


SPM.xsDes.Basis_functions - type of basis function

details on scannerdata (e.g. smoothness) 


SPM.xVol - structure containing details of volume analyzed
SPM.xVol.M- 4x4 voxel --> mm transformation matrix
SPM.xVol.iM - 4x4 mm --> voxel transformation matrix
SPM.xVol.DIM - image dimensions - column vector (in voxels)
SPM.xVol.XYZ - 3 x S vector of in-mask voxel coordinates
SPM.xVol.S- Lebesgue measure or volume (in voxels)
SPM.xVol.R- vector of resel counts (in resels)
SPM.xVol.FWHM - Smoothness of components - FWHM, (in voxels)

info on beta files:


SPM.Vbeta - struct array of beta image handles
SPM.Vbeta.fname - beta img file names
SPM.Vbeta.descrip - names for each beta file

info on variance of the error


SPM.VResMS - file struct of ResMS image handle
SPM.VResMS.fname - variance of error file name

info on mask


SPM.VM - file struct of Mask image handle
SPM.VM.fname - name of mask img file

contrast details (added after running contrasts)


SPM.xCon - Contrast definitions structure array
  • (see also spm_FcUtil.m for structure, rules &handling)
SPM.xCon.name - Contrast name
SPM.xCon.STAT - Statistic indicator character ('T', 'F' or 'P')
SPM.xCon.c - Contrast weights (column vector contrasts)
SPM.xCon.X0 - Reduced design matrix data (spans design space under Ho)
  • Stored as coordinates in the orthogonal basis of xX.X from spm_sp
  • (Matrix in SPM99b)
  • Extract using X0 spm_FcUtil('X0',...
SPM.xCon.iX0 - Indicates how contrast was specified:
  • If by columns for reduced design matrix then iX0 contains the column indices.
  • Otherwise, it's a string containing the spm_FcUtil 'Set' action: Usually one of {'c','c+','X0'} defines the indices of the columns that will not be tested. Can be empty.
SPM.xCon.X1o - Remaining design space data (X1o is orthogonal to X0)
  • Stored as coordinates in the orthogonal basis of xX.X from spm_sp (Matrix in SPM99b) Extract using X1o spm_FcUtil('X1o',...
SPM.xCon.eidf - Effective interest degrees of freedom (numerator df)
  • Or effect-size threshold for Posterior probability
SPM.xCon.Vcon - Name of contrast (for 'T's) or ESS (for 'F's) image
SPM.xCon.Vspm - Name of SPM image